1. Field of the Invention
The principles of the present invention relate generally to an improved telecommunication system, and in particular, to an improved method of testing a communication device. Still more particularly, the principles of the present invention relate to a method and apparatus for creating a real loop for measuring sidetone levels in communication devices.
2. Description of the Related Art
Several electrical circuits operate over large distances. For example, telephone networks, such as public switched telephone network (PSTN), connect telephone devices to the telephone network infrastructure components over several hundred or thousand feet. A typical analog phone is connected to a central office (CO), a location in the telephone network that houses telephone equipment, and provides telecommunication services using the analog phone. Typically, the distance between the telephone device and the CO is of the order of several thousand feet. An electrical cable spans that distance to electrically connect the telephone device to the CO via a number of intermediate devices. Two wires of the electrical cable, one considered running from the telephone device to the CO, and the other considered running from the CO to the telephone device, form a loop. A loop is an electrically conductive path starting from a telephone device, passing through the CO, and terminating at the same telephone device.
In most telephone conversations, a telephone device sends or receives three types of signals. First, a transmit signal which is an electrical signal corresponding to the voice of a user speaking into the microphone of the telephone device. Second, a receive signal which is an electrical signal corresponding to the voice the user hears via the speaker of the telephone device. Finally, a sidetone signal, which is an electrical signal corresponding to the user's own voice picked up by the telephone's microphone, that is fed back to the telephone's speaker via the CO to enable the user to know that the phone is working.
Sidetone is produced by the transmit signal and provides the user a confirmation of the speech level, or pitch, and quality of the transmitted signal. Strength of the sidetone signal is used in maintaining the quality of the call. For example, if the sidetone signal strength is zero, a user might get an impression that the telephone connection is non-functional. If the sidetone signal strength is below a certain threshold level, the user might think that the other user may not be able to hear the user's voice loud enough and consequently speak loudly. If the sidetone signal strength is higher than the threshold, the user might think that the user is talking too loudly and consequently might talk too softly for the other user to be able to hear. Sidetone signals of higher than certain threshold signal strength can also cause ambient noise to be picked up together with the user's voice, thereby making for a noisy connection.
Presently, a telephone equipment is tested for the telephone equipment's response to the sidetone signal of various strengths. In order to simulate the distance the sidetone signal travels—from he telephone equipment's microphone, to the CO, to the telephone equipment's speaker—the test uses a simulation circuit that includes resistors, capacitors, and inductors to simulate the impedance of the length of the wire covering that distance. The sidetone signal strength varies as the distance between the telephone equipment and the CO changes. The tests vary the impedance of the simulation circuit to simulate the various distances. However, the simulation circuits may not realistically reflect the characteristics of an actual length of wire traversing the distance to and from the CO. Consequently, the sidetone signal strength tests using the simulation circuit may not adequately reflect the telephone equipment's response to an actual sidetone signal.